The present invention relates generally to the control of irrigation water, and specifically to automatic irrigation systems utilizing soil-moisture tensiometers.
Tensiometers are pressure-measuring devices for measuring the matric tension (suction) of water in soil. This tension is a function of the quantity of water in a given soil sample, and depends also on the type of soil and its condition. Stated otherwise, this matric suction is a measure of how tightly the water is held to the soil particles and how available it is to plants.
Several types of soil tensiometers are known. Generally they include a tube hermetically sealed at the top, closed at the bottom by a water permeable body (e.g., porous ceramic), and often have a water refilling device. The tube is filled with deaerated water, and the water-permeable end is inserted into the soil with the tube supported in a vertical or inclined position. Water within this tube passes through the water-permeable body in either direction (i.e., into or out of the ground) in accordance with the matric tension (suction) of the soil, causing the pressure in the tensiometer to drop below the reference or ambient pressure, which is usually atmospheric pressure.
If the soil dryness is such that the water continues to escape from the tensiometer, at some point the pressure in the tensiometer falls to a level at which air enters the tensiometer, typically at -80 to -100 centibars with respect to atmospheric pressure (below atmospheric pressure). Then the tensiometer must be recharged with water for continued operation.
An example of a known automatic irrigation flow controller for use with a tensiometer is taught in U.S. Pat. No. 3,910,300 issued to Tal on Oct. 7, 1975. It employs a diaphragm as a pressure sensor, has a large number of parts, and requires electricity or a separate pilot valve for switching ON or OFF the flow of water for irrigation. The rate of irrigation flow is not variably controlled by the pilot valve, a separate main valve being referred to apparently for setting any specific quantity of flow.